Type II Diabetes, Obesity, and Breast Cancer Risk: The Multiethnic Cohort.

Obesity has been more consistently associated with breast cancer than type II diabetes. This analysis examined the combination of the two factors in the Multiethnic Cohort (MEC). Women ages 45-75 years entered the MEC in 1993-1996 by completing a questionnaire. Type II diabetes status was self-reported at baseline, two follow-up questionnaires, and confirmed by administrative data. Cancers were identified from tumor registries and deaths through vital records until 2010. Cox regression was applied to estimate HRs and 95% confidence intervals (CI) for BMI and type II diabetes status alone and in combination. Among 103,721 (25,146 white, 20,255 African American, 7,681 Native Hawaiian, 28,012 Japanese American, 22,627 Latina) women with 14,558 type II diabetes cases, 6,692 women developed breast cancer during 14.8 ± 4.1 years of follow-up. Type II diabetes was significantly associated with breast cancer risk (HR, 1.15; 95% CI, 1.07-1.23), but including body mass index (BMI) lowered the HR to 1.08 (95% CI, 1.00-1.16). Ethnic-specific BMI-adjusted models showed elevated risks for type II diabetes in Latinas only (HR, 1.30; 95% CI, 1.11-1.52). In contrast, obesity predicted statistically significant 21%-46% higher risks, after type II diabetes adjustment, in all ethnic groups except Latinas (HR, 1.17; 95% CI, 0.99-1.38). As reported previously, inclusion of BMI weakened the association of type II diabetes with breast cancer. Type II diabetes status, but not BMI, was primarily associated with higher breast cancer risk in Latinas. The role of obesity and type II diabetes in breast cancer etiology may differ by ethnicity suggesting metabolic differences related to obesity. .